Aerial lift devices are often used to service sign posts, wires on telephone poles and the like. These devices usually include a boom with a bucket at one end. An operator standing in the bucket is able to control the movement of the boom. Many aerial lift devices include winches at the end of the boom adjacent to the bucket to lift working objects such as transformers and signs to elevated work areas. The working object is attached to a winch cable, and the bucket and working object are simultaneously lifted by the boom.
Each aerial lift has a limited lifting capacity. When this capacity is exceeded, the lift becomes unstable and is subject to tipping over. Therefore it is helpful for an aerial lift to have a method of measuring the load on a winch connected to the boom. Prior art devices measure the pressure in hydraulic cylinders used to move the boom. However, this pressure measurement cannot be directly converted into an accurate weight measurement because the pressure of a hydraulic cylinder varies with the elevational angle of the boom. Moreover, even if an accurate measurement were possible, a load measurement based on hydraulic cylinder pressure measures the load on the entire boom, instead of the load on just the winch.
Other lifting devices, such as cranes, use tensiometers to measure the load on the end of a jib. A tensiometer measures the tension of a cable, which can be directly converted to a measurement of the load on the cable. The tensiometer is located in the cab of the crane so it is visible to the operator. This arrangement is not helpful for aerial lift devices for two reasons. First, winch cables on aerial lifts do not traverse the length of the boom as with cranes. Second, the operator of an aerial lift is located in the bucket at the end of the boom, and is thus unable to read tensiometer located at the opposite end of the boom. Therefore the present state of the art provides no method for the operator of an aerial lift device to obtain a quick and accurate measurement of the load on a winch located at the end of a boom.